The U.S. gets a 63. Criteria include economic benefits of coastal waters, along with aesthetics and environmental data.

After two years of collecting global data and developing models, scientists have a new, comprehensive way to measure the health of the world's oceans that recognizes humans as a part of an integrated marine ecosystem.

The scientists' report, published this week in the journal Nature, gave the oceans an overall score of 60 on a scale of 0 to 100. Among the world's 133 countries with ocean coastlines, scores ranged from 36 to 86; the United States scored slightly above average at 63.

The ocean health index measures 10 ways that people benefit from the oceans, including food, jobs, ability to sequester carbon from the atmosphere and pure aesthetic value. It also gives credit for clean waters and biodiversity, among other things.

The index score assigned to a particular ocean region reflects the degree of sustainability for each of these factors.

To come up with scores for each country, a group of more than 30 scientists used data from dozens of sources. They got economic data from the United Nations, for instance, and satellite data on ocean temperature, sea ice extent and UV radiation from the U.S. National Oceanic and Atmospheric Administration. The 133 regions were defined as exclusive economic zones that extend from a country's shore to a boundary 200 nautical miles out to sea.

One way to think about the ocean index score is to compare it to a hospital visit.

"When someone shows up at the ER, there are things people look at: breathing, heartbeat, pulse," said study coauthor Larry Crowder, science director of the Center for Ocean Solutions at Stanford. The criteria picked for this study are like vital signs for the ocean, he said.

Previous ecosystem assessments focused on ways humans have damaged nature, such as by polluting waterways or driving species to the brink of extinction. For this index, researchers decided to award points for the ways that oceans could sustainably benefit people, even though such benefits might come at the expense of another goal.

For example, an increase in coastal livelihoods and economies might decrease a region's score for clean waters but still boost the index score overall. By quantifying these trade-offs, the ocean health index can help countries do their own cost-benefit analyses depending on what they value most.

"The old model of trying to save nature by keeping people out simply won't work," said study coauthor Steven Katona, managing director of the Ocean Health Index for the nonprofit environmental group Conservation International based in Arlington, Va. "People and nature are not separate anymore."

One of the biggest questions raised by the study is what to make of a score like 60.

To some, that may sound like an unsatisfactory grade, a D-minus. But project leader Benjamin Halpern, director of the Center for Marine Assessment and Planning at UC Santa Barbara, said that would be the wrong way to look at it.

"There is a lot of room for improvement, but still a lot of success," he said. "It wasn't a 10 or 15."

The index score for the United States suggests, according to the Nature study, that the country could improve its ocean health by supporting tourism businesses that are environmentally friendly; encouraging sustainable fishing practices; and investing in aquaculture to provide jobs and economic benefits to coastal communities.

Boosting the global index score rests mainly on improving sustainable fishing and cultivation of marine organisms for food provision as well as halting the loss of coastal habitats, the study found.

Generally speaking, higher-income countries had higher scores thanks to better environmental regulations and more money to devote to ocean-friendly pursuits. But there were several exceptions, most notably Singapore (which scored a 42) and Poland (a tad better at 48). Both countries had low levels of food provision from the ocean and fewer "lasting special places," among other problems.

The researchers said they plan to recalculate the global score annually.

Fiorenza Micheli of Stanford, who studies the ecology of marine communities and was not a member of the research team, said the new ocean health index would be "extremely useful" to policymakers and interest groups and could help spark a public dialogue about the oceans' health.